In a frequency hopping (or channel hopping) mesh network, devices communicate using different frequencies/channels at different times. To communicate a packet, a transmitter-receiver pair must be configured to the same channel during packet transmission. For a transmitter to communicate with a receiver at an arbitrary time in the future, the transmitter and receiver must synchronize to a channel schedule that specifies what channel to communicate on at what time. That is, to communicate a message, the transmitter must transmit the message according to the receiver's frequency hopping schedule. Note that the transmitter will often need to transmit the message on a channel different from its own receiving schedule.
Many communication devices, particularly in low-power and lossy network (LLNs), typically communicate using only a single transceiver due to cost or energy constraints. Most narrow band transceivers in use today can only be configured to transmit or receive on a single channel at a time. That is, while configured for a particular channel, the transceiver cannot transmit or receive on any other channel, and since transceivers are typically half-duplex, they do not have the ability to transmit and receive at the same time.
Because a narrow-band transceiver is limited to a single channel and is half-duplex, determining the cause of a packet drop effectively in a carrier sense multiple access (CSMA)-based network can be challenging. In particular, when a device “A” fails to receive an acknowledgment from a device “B,” it does not know if the transmission was lost due to link quality issues or simply because device B was tuned to a different channel and/or transmitting at the same time.
It could be argued that transmissions while the intended receiver was busy should be included in a link reliability metric. However, doing so conflates link reliability with link congestion and utilization. It is known that the ability to separate out actual transmission failures from congestion can significantly increase system performance, especially in a lossy environment such as LLNs. In particular, searching for a different route may best solve link reliability issues, while on the other hand, simply slowing down the transmission rate to reduce congestion may solve congestion issues.